This invention relates to a field emission display, more particularly to a method of improving field emission efficiency of carbon nanotube field emitters by using a taping film.
Nanotube field emission display comprises an image pixel array formed on a substrate having conductive patterned thereon as a cathode, and a corresponding phosphor pattern coated on an ITO glass as an anode. Each image pixel contains carbon nanotube (hereinafter called CNT) layer thereon as electron emission sources. The CNT layer made of a slurry consists of organic bonding agent, silver powder, and CNT, which having 5-100 nm in diameter and 1000-3000 nm in length. The principle of field emission is in terms of electric field accelerating cold electron which is emitted from the tip of CNT through vacuum space and bombards anode which is an indium tin oxide (ITO) substrate having phosphor pixel to generate fluorescence. By contrast to conventional cathode ray tube which is in terms of thermionically emitted electrons emerge from a tungsten wire, the field emission modeling has quite different fashion.
A typical field display schematic cross-sectional view is shown in FIG. 1. The figure shows a conductive line array 20 coated on a substrate 10 by screen-printing a conductive slurry containing silver through a line-patterned screen. Thereafter, a CNT layer is attached thereon by screen-printing a CNT paste through a mesh patterned screen to form image pixel layer 30. The CNT paste consists of organic bonding agent, resin, carbon nano-tubes, and silver powder. After that the substrate is soft baked in an oven using a temperature of about 50-200xc2x0 C. to remove volatile organic solvent. Finally a higher temperature sintering process is carried out to cure the CNT on and to electric coupled with the conductive silver lines. In the sintering process, all of the organic bonding agent and resin are burned out.
The cost of the above fabricating process is low and it is not a mere benefit. Field emission display can have very thin electron gun of only about 0.2 mm in thickness. In addition, the size of planar area can have very flexible, it can be very small such as 1 cm2 and can as large as several hundred centimeter square. The CNT-FED is thus a prominence for an ultra thin flat panel display. For a display, the stability and long life reliability are fundamental characteristics. However, as high as 10-100 mA/cm2 in current density is a minimum criteria and the most critical characteristic for generating sufficient brightness and uniformity for a display. To approach such current density, the lower intensity electrical field is preferred. Preferably, it should be lower than 5 V/xcexcm.
Since the electric property (current density vs. intensity of electric field) is predetermined by a number of exposed CNT, which should be electric coupled with the conductive layer 20 of the cathode. However aforementioned CNT field emission device of prior art in general emits very low current density unless using extra processes and/or using high electric field intensity. Please refer to FIG. 2, showing a curve 110 by using conventional process and another curve 120 in accordance with the present invention. In figure, the current density versus electric field is shown. The conventional process has a current density lower than 1 mA/cm2 for intensity of electric filed of about 6V/xcexcm. To achieve 10-100 mA/cm2 in current density emission exerting rather high electric field intensity is usually expected.
Thus, as acquired knowledge known by the inventor, none of issued invention discloses a CNT emission display, which can approach the goal of producing the critical current density in the electric field intensity as low as 6V/xcexcm. U.S. Pat. No. 5,616,368, issued to Jim, et al., disclose a patent about field emission display. Jim, et al. proposed that using activated ultra-fine diamond particulate as emission sources for field emission source can significantly improving the prior art of their patent. As stated in Jim""s patent, ultra-fine diamond particulate has a low or negative electrical affinity, and thus can act as field emitter in low electric field. An electric field of more than 70V/xcexcm is needed for typical p-type doped diamond substrate to generate an emission current density of 10 mA/cm2. In Jim""s patent, a field of a bout 12V/xcexcm or even down to 5V/xcexcm is required to achieve the critical current density.
The method of Jim, et al comprises the following steps. First, diamonds, predominantly having maximum dimensions in the range of 5-10,000 nm are prepared. Prior to paste the particulate emitters to the substrate, the ultra-fine particles are exposed in a plasma containing hydrogen at a temperature in excess of 300xc2x0 C. In order to minimize agglomerations of the particles during the plasma activating processing and in order to have relative uniform activation on major part of the exposed diamond surface, the particles in continuous motion by injecting high speed gas flow is performed. In addition, the diamond particles have less than 10 volume percent of graphitic or amorphous carbon phases. Thereafter the diamonds particles with bond agent are mixed and screen-print to a predetermined conductive trace containing substrate. Finally, a sintering process at a temperature of about 500xc2x0 C. is performed to form pixels.
As forgoing prior art, for CNT there is not available method present to improve the problem of the high electric filed needed unless using the ultra-fine diamond particles. In addition, the method to alleviate agglomeration of the particles proposed by the prior art is done agglomerated again after slurry prepared and sintering process is not sure. Thus the present invention is to improve the CNT field emitter. Furthermore, CNT field emitter improvement by the present invention has lower cost and easily to implement.
The present invention is to propose a simple CNT field emitter forming method, which significantly improves the efficiency of CNT.
The present invention discloses a method of CNT emitter current density improvement by a taping process. The method comprises following steps. First of all, a conductive pattern coated on a substrate by screen-printing a conductive slurry containing silver through a patterned screen is carried out. Thereafter, a CNT layer is attached thereon by screen-printing a CNT paste through a mesh pattern screen to form CNT image pixel array layer. The CNT paste consists of organic bonding agent, resin, silver powder, and carbon nano-tubes. After that the substrate is soft baked by an oven using a temperature of about 50-200xc2x0 C. to remove volatile organic solvent. A higher temperature sintering process, for example 350-550xc2x0 C. is then carried out to solidify the CNT on and electric coupled with conductive pattern. Finally, an adhesive film is closely attached on the cathode substrate and is then removed the adhesive film away so as to remove those badly bonding CNT portion and to vertically pull up a portion of CNT which originally laid down on the surface after sintering. Consequently, the current density, brightness, and uniformity of the emitter sources are significantly improved.